The present invention relates generally to an imaging system utilizing imaging sheets having a surface coating of rupturable photosensitive microcapsules and, more particularly, to a method and apparatus for controlling the exposure of the imaging sheets for more precise image reproduction.
U.S. Pat. Nos. 4,440,846 and 4,399,209, hereby incorporated by reference, describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is imagewise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and imagewise release the internal phase. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet developing processing solutions to produce the image. An image-forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former imagewise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing imagewise exposed imaging sheets through the nip formed between a pair of parallel calendar rolls.
The media may exist in either single-sheet or two-sheet versions. In the former case, the microcapsules and developer composition are both coated onto a single substrate layer. In the latter case, the microcapsules are carried on a first substrate layer referred to as a donor sheet. The developer composition is coated onto a second, separate substrate layer referred to as a receiver sheet. The donor sheet is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheet. The two sheets are then developed by pressure, with the finished image being formed in the receiver sheet.
The disclosed imaging system may be designed to produce color images. In such case, three or more different types of capsules may be present on the sheet. Each is responsive to a different wavelength of exposure radiation and contains color formers designed to produce different colors upon subsequent development. Disclosure of such systems is made in U.S. Pat. No. 4,576,891 and U.S. Pat. application Ser. No. 339,917 filed Jan. 18, 1982.
U.S. Pat. application Ser. No. 262,545 filed Oct. 25, 1988, discusses a phenomenon exhibited by the imaging media known as short time scale reciprocity failure. Absent this effect, it would be expected that the imaging media would exhibit a substantially linear relationship between exposure time and exposure intensity. However, at short exposure times, the media exhibits a dramatic loss in sensitivity. As noted in the application, this effect can be greatly diminished by lowering the temperature of the imaging media as it is exposed.
The referenced application also notes that variation in media temperature can affect sensitometric behavior of the media such as film speed, contrast, dynamic range and the like. Thus, the media and/or an imaging apparatus with which the media is to be used may be designed to correspond to characteristics of the media exhibited at a particular temperature. As temperature varies from an optimized value, the quality of image produced on the media or by the apparatus may be degraded.
Similar effects may be observed due to variations in relative humidity. The media and/or the imaging apparatus may be designed to operate optimally at a particular relative humidity, with variations in humidity affecting the quality of the image produced.
A further problem is encountered when using a color media. The different microcapsules corresponding to different color formers exhibit varying degrees of temperature and/or humidity related changes. Thus, the media may be designed to operate at a particular optimum temperature and/or humidity level, whereat the different color microcapsules exhibit predictable behavior. As temperature and/or humidity changes, the sensitivity of the different microcapsules may change to different extents, thereby affecting the color balance or perceived hue of the color image.